Add support for coverage information

[niknetniko]

We could add coverage information, which can be useful, mainly for teachers. For example, we teachers can see if their tests cover all code in the submission from the student, or find superfluous code in the sample submission.

[pdawyndt]

Different ways to measure code coverage:

• line coverage: how many of the lines in the submission have been executed
• statement coverage: how many of the statements in the submission have been executed
• branch coverage: how many of the branches of the control structures (if statements for instance) have been executed
• condition coverage: how many of the boolean sub-expressions have been evaluated for a true and a false value